With vibronic measuring devices, a large number of physical variables can be detected or monitored, such as, for example, the fill level, the phase boundary, the density or the viscosity of a medium in a container or in a pipeline. The Endress+Hauser Group provides such measuring devices in a large variety under the marks, LIQUIPHANT and SOLIPHANT.
Vibronic measuring devices have an oscillatable unit, which, most often, includes a membrane and an oscillatory rod secured thereon or an oscillatory fork having two tines. There are, however, also vibronic measuring devices known, whose osciliatable unit is only the membrane.
The exciting of the oscillatable unit to oscillate with the resonance frequency occurs usually via a piezoelectric stack- or bimorph drive or via electrodynamic drive elements, wherein the drive unit, most often, is embodied as a combined transmitting/receiving unit. The transmitted signal and the received signal have a phase shift, which usually lies in a defined range around 90°.
For fill level measurement, the effect is utilized that the oscillation frequency and oscillation amplitude change, when the degree of covering of the oscillatable unit changes. In air, undamped oscillation occurs, while oscillation is damped in the case of an oscillatable unit covered with medium. This change in the oscillation amplitude or oscillation frequency is utilized to detect the reaching of a predetermined fill level, usually a maximum fill level, for overflow protection or a minimum fill level for protection against running empty.
The oscillation amplitude of an oscillatory fork or of an oscillatory rod depends furthermore on how much mass is being dragged. If the density or the phase of a medium changes, then this influences the oscillation of the oscillatable unit, so that a transition between different phases as well as also density changes can be detected with the vibronic measuring device. In a similar manner, the oscillation is damped when the viscosity of the medium increases.
In order to so set the exciter frequency that the oscillatable unit oscillates with a predetermined phase shift between transmitted signal and received signal, the Endress+Hauser Group has developed a method utilizing a frequency search sweep. A corresponding German patent application (Application No. DE 102009026685) has been filed but has not yet been published. The method will be described briefly here.
In the frequency search sweep, the oscillatable unit is excited successively to oscillate with discrete, closely neighboring frequencies and the frequency corresponding to the predetermined phase shift is ascertained. The ascertaining of the frequency occurs, in such case, via phase selective rectification and following low-pass filtering. A rectangular signal delayed by the predetermined phase shift relative to a received signal is multiplied with the received signal. If the phase shift between the transmitted signal and the received signal corresponds to the predetermined phase shift, then this product contains only positive portions, while in the opposite case, it would contain negative portions. The described evaluation methods can be implemented analogly or digitally. The embodiment with analog components has, in such case, two disadvantages. On the one hand, the circuit complexity is high and, on the other hand, the determining of the oscillation frequency at the predetermined phase shift is relatively inaccurate. The advantages in the case of the digital practice of the method include that the high circuit complexity is absent and, additionally, the oscillation frequency can be determined with high accuracy. The disadvantage of the digital method is, however that it is very calculation intensive, which makes a high computing power necessary.